Wafers, such as semiconductor wafers, can become bowed. This renders the surface of the wafer curved (i.e., non-planar). For example, a point on the surface of a bowed wafer can deviate from a reference plane relative to a circumference of the surface. Such bowing may be a result of wafer processing or a result of stress or strain to the wafer. For example, layers or films on the wafer can cause stress or strain that leads to bowing. Bowing also may be a temporary effect due to, for example, chucking in wafer processing equipment.
Recently, edge grip chucks have increased in prevalence. Some manufacturers have sought to avoid using vacuum or electrostatic chucks when devices are formed on both flat surfaces of a wafer because the devices may contact the vacuum or electrostatic chuck and become damaged during chucking. Edge grip chucks avoid contact with most or all of the back side of a wafer by gripping the circumferential edge of the wafer. However, holding the wafer along its circumferential edge can lead to wafer sagging because the back side of the wafer is unsupported. This sagging can exacerbate bowing of the wafer due to stress or strain caused by the various layers or films formed on the wafer.
Inspection processes are routinely used during wafer processing to determine whether, for example, devices are being properly formed or whether defects exist on the wafer. Manufacturers may inspect wafers at multiple points during the manufacturing process. Early identification of defects can reduce manufacturing costs because time and resources are not spent processing a wafer with defects or non-functional devices. This inspection is typically done optically using an illumination source to project light on the wafer and a sensor to capture the reflected (brightfield) or scattered (darkfield) signal.
Inspection of a wafer surface is made challenging by process variations, such as wafer bowing. An optical system's ability to focus over the area of a surface is limited by its depth of field, or the axial depth of space on both sides of the focal plane within which objects appear acceptably sharp. In the case of wafer inspection, an image of devices on a wafer surface may be considered to have acceptably sharp focus when device defects can be resolved. When an optical system is focused on a portion or segment of the wafer, the wafer surface within the field of view of the optical system may not be in focus in its entirety. If a segment of the wafer surface in a field of view, or part of the wafer surface visible to the optical system at a particular position and orientation, is not within the depth of field of the optical system, portions of the image outside of the depth of field will not be in focus and, therefore, the optical system will not be able to produce a meaningful image of the wafer surface for purposes of inspection.
The ramifications of incomplete or improper inspection can be drastic. For example, while photoresist can be reworked if inspection determines the pattern to be incorrect, incomplete inspection may cause photoresist defects to go undetected. It may be impossible to make corrections once a wafer is etched or implanted. In that case, the wafer must be scrapped. As such, early detection of defects may allow for cost and time savings.
Previous methods of compensation for bowing during inspection, such as decreasing the imaging field of view (potentially requiring an increase in the number of sensors), are insufficient. Therefore, what is needed is a technique for inspection of bowed wafers. More particularly, what is needed is a system and method for inspection of bowed wafers that does not sacrifice sensitivity or throughput.